The invention relates to the field of video signal processing, and, more particularly to an improved device and method for digitally separating a composite video signal into luminance and chrominance components.
According to Grassman's laws, the human eye can distinguish three kinds of differences or variations. These three components can be represented in many ways. One common way is to represent them as intensities of red (R), green (G), and blue (B) light. They can also be represented as a single luminance (Y) component, along with two chrominance (C) components. The chrominance components can be viewed as color difference signals (B-Y, R-Y), or in polar coordinates as an angle (hue) and magnitude (saturation.)
Typically, composite video signals are generated by adding a baseband luminance signal to a quadrature modulated chrominance signal. Two commonly used standards are known as “NTSC” (developed by the National Television System Committee in the USA in 1953) and “PAL” (developed in Europe in the 1960's, employing the technique of phase-alternating lines.) There are several variations of these techniques, including the use of different line and frame frequencies and different sub-carrier frequencies used for quadrature modulation. Details of such techniques are well known and can be found in the relevant national standards, or in international standards such as ITU-R BT.470. The theory about the components of visible light and the techniques used for generation of composite video signals are also well known and can also be found in any textbook on video signal processing, such as Digital Video and HDTV Algorithms and Interfaces, by Charles Poynton.
The simplest way to separate the luminance and chrominance components of a composite video signal is to use a combination of low-pass and high-pass filters. This technique assumes that the majority of the luminance signal is below a certain frequency, while the majority of the chrominance signal is above the same frequency. Because the filters only operate in the horizontal direction of the image, this technique is considered one-dimensional. A block diagram of a device employing this technique appears in FIG. 1. The composite signal 100 goes through a low-pass filter 101 to generate the luminance signal 102. The composite signal 100 also goes through a high-pass filter 103 to generate the modulated chrominance signal 104. The modulated chrominance signal 104 is then demodulated by a quadrature demodulator 105 to form the color difference signals 106, 107.
The disadvantage of this configuration is that, in practice, there is some overlap between luminance and chrominance signals in the frequency domain. Therefore, this structure will cause some luminance information to be decoded as chrominance, and vice versa, resulting in visible artifacts in the decoded image. The restriction of the luminance bandwidth to lower frequencies also results in a lower quality image.
Another well known configuration for separating luminance and chrominance recognizes that the luminance and chrominance signals typically do not change a great deal between adjacent lines, and the chrominance subcarrier is designed to have opposite phase for either adjacent lines (NTSC) or every second line (PAL.) Thus, by averaging the composite signal for two lines, the modulated chrominance signal will cancel, leaving only the luminance signal. By taking the difference of two lines, the luminance signal will be cancelled, leaving only the chrominance signal. This type of structure is known as a comb filter, due to the resemblance of its frequency response to the teeth of a comb. Because this structure operates in both horizontal and vertical directions of the image, it is considered a two-dimensional technique. A block diagram of a structure employing a simple comb filter appears in FIG. 2. The composite signal 100 goes through a delay element 201, which delays the signal by one (NTSC) or two (PAL) lines. The delayed signal 202 goes to an adder 203, where it is added to the composite signal 100 to obtain the luminance component 204. The delayed signal 202 also goes to a second adder 205, where it is subtracted from the composite signal 100 to obtain the modulated chrominance component 206. Scale factors may be required before or after the adders to ensure that the component signals are in the correct range. The modulated chrominance signal 206 is then demodulated by a quadrature demodulator 207 to form the color difference signals 208, 209.
One problem with the simple comb filter structure of FIG. 2 is that the assumption that luminance and chrominance signals do not change substantially between lines is not always true. Therefore, the decoded image will have visible artifacts around horizontal (line to line) transitions. Another disadvantage of this structure is that it is sensitive to errors in the phase of the chrominance signal. If the phase of the chrominance subcarrier is not exactly opposite after the delay, it will result in imperfect cancellation of the chrominance signal.
To address the problem of horizontal transitions, more complicated signal separation devices use signal information from three or more lines, and employ a vertical processing block to detect transitions and select various combinations of the lines based on that detection. The vertical processing block may also select a horizontal filter output when an appropriate combination of lines cannot be found. An example of this type of structure can be found in UK Patent Application GB 2066615, “Improvements to Colour Television Decoding Apparatus.”
Further improvements can be realized by using signal information from multiple frames of the image. Because these structures operate in horizontal and vertical directions of the image, as well as over multiple frames, they are considered three-dimensional. Examples can be found in U.S. Pat. No. 5,473,389, “Y/C Separator Using 3-D, 2-D, and 1-D Filters,” and U.S. Pat. No. 5,502,509, “Chrominance-Luminance Separation Method and Filter Performing Selective Spatial Filter Based on Detected Spatial Correlation.” A generalized block diagram of the enhanced comb filter structure, which may be two-dimensional or three-dimensional depending on whether any of the delay elements store entire frames, appears in FIG. 3. Hereafter, the term “vertical processing” will be used to mean two-dimensional and/or three-dimensional processing. The composite signal 100 goes to a cascade of delay elements 301, which delay the signal by various multiples of the line or frame period, generating multiple delayed signals 302. Although only two delay elements are shown in the figure, more could be added without departing from the general structure. The delayed signals 302 and the composite signal 100 proceed to a vertical processing block 303, which determines the best combination of signals to generate luminance 304 and modulated chrominance 305 signals. The modulated chrominance signal 305 is then demodulated by a quadrature demodulator 306 to form the color difference signals 307, 308.
In all of the configurations described above, the signal separation operations occur before quadrature demodulation of the chrominance signal. These can be called “passband” structures. To address the problem of subcarrier phase sensitivity, some signal separation devices perform demodulation before the vertical processing block. These can be called “baseband” structures. Examples of such structures are U.S. Pat. No. 6,052,157, “System and Method for Separating Chrominance and Luminance Components of a Color Television System;” U.S. Pat. No. 6,175,389, “Comb Filtered Signal Separation;” and U.S. Pat. No. 6,459,457, “Adaptive Color Comb Filter.” A generalized block diagram of the baseband comb filter structure appears in FIG. 4. The composite signal 100 goes to a composite delay element 401, which generates a delayed composite signal 402. The composite signal 100 also goes to a quadrature demodulator 403, which generates a complex baseband signal 404. The complex baseband signal 404 goes to a horizontal processing block 405, which generates a filtered complex baseband signal 406. The filtered complex baseband signal 406 goes to a cascade of complex baseband delay elements 407, which delay the complex baseband signal by various multiples of the line or frame period, generating multiple delayed complex baseband signals 408. Although only two baseband delay elements are shown in FIG. 4, more could be added without departing from the general structure. The delayed complex baseband signals 408 and the filtered complex baseband signal 406 proceed to vertical processing block 409, which determines the best combination of signals to generate first color difference signals 410, 411 and second color difference signals 416, 417. The first color difference signals 410, 411 are used for output. The second color difference signals 416, 417 proceed to a remodulator 412, which generates a modulated chrominance signal 413. The second color difference signals 416, 417 may or may not be the same as the first color difference signals 410, 411. The modulated chrominance signal 413 goes to adder 414, which subtracts the modulated chrominance signal 413 from delayed composite signal 402 to form the luminance output 415.
The main disadvantage of this configuration is that it requires increased memory space to implement the complex baseband delay elements. Because the baseband signal is complex, it requires twice the memory as the composite signal, assuming the same precision and sampling rate requirements. This is because the composite signal has only a real part, whereas the complex baseband signal has both real and imaginary parts. This requirement may be reduced by decimating or reducing the precision of the complex baseband signal. For example, see FIG. 10 of U.S. Pat. No. 6,175,389 and the relevant description. However, both decimation and precision reduction result in the loss of signal information that may be useful for later processing. Decimation of the baseband signal also requires that interpolation be done before remodulation, increasing the complexity of the implementation.
Therefore, there exists a need in the art for an optimized signal separation structure to implement baseband signal separation with reduced memory requirements, and without loss of signal information. As will be understood below, the invention accomplishes this in an elegant manner.